Almost every galaxy in the universe has at its core a supermassive black hole. Or, more accurately, we should say that they have at least one because an international team of researchers has just discovered a galaxy that has three for the first time.
Galaxy NGC 6240 has an irregular shape due to being the end product of a galaxy merger. The assumption has been that two galaxies collided long ago due to the presence of two supermassive black holes. But, as reported in the journal Astronomy & Astrophysics, new observations have revealed it has three, not two.
“Through our observations with extremely high spatial resolution we were able to show that the interacting galaxy system NGC 6240 hosts not two – as previously assumed – but three supermassive black holes in its centre,” lead author Professor Wolfram Kollatschny from the University of Göttingen said in a statement.
Each of the three supermassive black holes has a mass over 90 million times of the Sun. By comparison, Sagittarius A*, the supermassive black hole at the center of the Milky Way is “only” just over 4 million times our Sun's mass. The three black holes are located in a volume of less than 3,000 light-years across.
“Such a concentration of three supermassive black holes has so far never been discovered in the universe,” adds Dr. Peter Weilbacher of the Leibniz Institute for Astrophysics Potsdam (AIP). “The present case provides evidence of a simultaneous merging process of three galaxies along with their central black holes.”
Galaxy mergers are uncommon but crucial events in the evolution of galaxies. It is the mechanism for the formation of the most massive galaxies in the cosmos. Astronomers are actually unsure how these formed in relatively short cosmic time. The existence of many multiple mergers such as NGC 6240 could explain it.
"If simultaneous merging processes of several galaxies took place, then the largest galaxies with their central supermassive black holes were able to evolve much faster,” Weilbacher explained. "Our observations provide the first indication of this scenario.”
While still apart, for now, the supermassive black holes will continue to move towards each other. Over the next million years, they will merge into a single one, releasing incredible gravitational waves.